Like a sore thumb: Niobium stands out among neighboring transition metals for its catalytic performance in the synthesis of cyclic carbonates. Readily available NbCl5 is a very efficient catalyst for the preparation of industrially relevant carbonates at ambient temperature, low CO2 pressure, and low concentrations. DMAP=N,N‐dimethylaminopyridine.

A mechanistic study on the synthesis of propylene carbonate (PC) from CO2 and propylene oxide (PO) catalyzed by NbCl5 and organic nucleophiles such as 4-dimethylaminopyridine (DMAP) or tetra-n-butylammonium bromide (NBu4 Br) is reported. A combination of in situ spectroscopic techniques and kinetic studies has been used to provide detailed insight into the reaction mechanism, the formation of intermediates, and interactions between the reaction partners. The results of DFT calculations support the experimental observations and allow us to propose a mechanism for this reaction.